Chemical liquids such as a photoresist liquid, a spin-on-glass liquid, a polyimide resin solution, purified water, a developing solution, an etching solution, and an organic solvent are used in the production processes in the technical fields including semiconductor wafer manufacturing techniques, liquid-crystal substrate manufacturing techniques, magnetic disk manufacturing techniques, and multi-layered wiring board manufacturing techniques; and chemical liquid supply apparatuses are used for application of the chemical liquids. For example, when a photoresist liquid is to be applied onto the surface of a semiconductor wafer, in a state in which the semiconductor wafer is placed on a horizontal plane and rotated, the photoresist liquid is dropped onto the surface of the semiconductor wafer. A chemical liquid tank is used as a device for containing the chemical liquid before dropping, and the chemical liquid tanks are differently used according to various objects.
For example, when a chemical liquid which has been filtrated through a filter is to be applied onto a wafer, in some cases, a filter is provided at the second side of a pump and the pump is actuated. At this point, the filtration speed and the discharge speed are the same. However, since the speed suitable for filtration and the speed suitable for application are normally different, the yield is sometimes reduced for the reason that, for example, air bubbles or gelled chemical liquid passes through the filter membrane. Thereat, there provided a case in which a filter is provided at the downstream of a first pump, a chemical liquid tank is provided in downstream of the filter, and a second pump is provided at the downstream of the chemical liquid tank, such that the first pump is subjected to a discharge operation, thereby temporarily storing the filtrated chemical liquid in the chemical liquid tank, and subsequently, the chemical liquid is sucked in by the second pump so as to apply it onto a wafer (for example, see, U.S. Pat. No. 5,490,765).
Meanwhile, when the replaceable chemical liquid tank (chemical liquid bottle) disposed at the uppermost streamside becomes empty, and when the chemical liquid supply apparatus is to be actuated even while the empty chemical liquid bottle is being replaced; there provided a case in which a chemical liquid tank for buffering is provided within the chemical liquid circuit, some of the chemical liquid in the chemical liquid bottle are stored therein in advance, and the chemical liquid contained in the chemical liquid tank for buffering is used as a substitute while the empty chemical liquid bottle is being replaced.
Meanwhile, in some cases, an application nozzle is provided about several meters above the chemical liquid tank disposed at the uppermost stream side of the chemical liquid circuit. In this case, when the chemical liquid in the chemical liquid tank is updrawn by one pump, the load imposed on the pump is excessive. Therefore, there provided a case in which an auxiliary tank and an auxiliary pump for relaying are provided within the chemical liquid circuit, such that the chemical liquid is updrawn to the auxiliary tank by the first pump, and subsequently, the chemical liquid in the auxiliary tank is updrawn to the application nozzle by use of the second pump.
When chemical liquid is dropped, the chemical liquid is reduced; therefore, the remaining amount of the chemical liquid in the chemical liquid tank has to be checked at appropriate timing. Until now, there known a method in which the remaining amount of the chemical liquid is checked by detecting the boundary face of air and chemical liquid, i.e., the level of the chemical liquid surface (liquid level) by a sensor. However, when a gas is mixed in the chemical liquid, the discharge amount becomes unstable since air bubbles absorb the supply pressure. Moreover, if the chemical liquid is the kind that is altered when it comes into contact with air, such as a photoresist liquid, the function of the chemical liquid is deteriorated, or the dropping amount of the chemical liquid becomes unstable, thereby deteriorating the yield of the product.